feat(02-01): install next-auth@4, configure CredentialsProvider auth

- Add next-auth@4 dependency (stable v4, not beta v5)
- Create src/lib/auth.ts with CredentialsProvider validating ADMIN_EMAIL/ADMIN_PASSWORD env vars
- Create src/app/api/auth/[...nextauth]/route.ts catch-all handler (GET + POST)
- JWT session strategy — stateless, no DB users table
- Custom sign-in page set to /admin/login
- Add NEXTAUTH_URL, NEXTAUTH_SECRET, ADMIN_EMAIL, ADMIN_PASSWORD to .env.local

src/app/api/auth/[...nextauth]/route.ts

@@ -0,0 +1,6 @@
+import NextAuth from "next-auth";
+import { authOptions } from "@/lib/auth";
+
+const handler = NextAuth(authOptions);
+
+export { handler as GET, handler as POST };

src/lib/auth.ts

@@ -0,0 +1,55 @@
+import type { NextAuthOptions } from "next-auth";
+import CredentialsProvider from "next-auth/providers/credentials";
+
+export const authOptions: NextAuthOptions = {
+  providers: [
+    CredentialsProvider({
+      name: "credentials",
+      credentials: {
+        email: { label: "Email", type: "email" },
+        password: { label: "Password", type: "password" },
+      },
+      async authorize(credentials) {
+        if (!credentials?.email || !credentials?.password) return null;
+
+        const adminEmail = process.env.ADMIN_EMAIL;
+        const adminPassword = process.env.ADMIN_PASSWORD;
+
+        if (!adminEmail || !adminPassword) {
+          throw new Error("ADMIN_EMAIL and ADMIN_PASSWORD env vars must be set");
+        }
+
+        if (
+          credentials.email === adminEmail &&
+          credentials.password === adminPassword
+        ) {
+          // Return minimal session user — no DB lookup needed
+          return { id: "admin", email: adminEmail, name: "Admin" };
+        }
+
+        return null; // null = unauthorized (NextAuth returns 401)
+      },
+    }),
+  ],
+  session: {
+    strategy: "jwt",       // stateless JWT — no DB session table (per D-03)
+    maxAge: 30 * 24 * 60 * 60, // 30 days
+  },
+  pages: {
+    signIn: "/admin/login",   // custom login page (per D-07)
+  },
+  callbacks: {
+    async jwt({ token, user }) {
+      if (user) {
+        token.id = user.id;
+      }
+      return token;
+    },
+    async session({ session, token }) {
+      if (session.user) {
+        (session.user as { id?: string }).id = token.id as string;
+      }
+      return session;
+    },
+  },
+};